When an insulated gate bipolar transistor (hereinafter, sometimes referred to as IGBT) is used in an inverter or the like, there is a mode where overvoltage and overcurrent are caused, which is called a load short. In order to avoid element destruction even when being caught in such a mode, it is desirable for a power device itself to have overvoltage and overcurrent resistance performance such as a function to detect abnormal heat generation due to overvoltage and overcurrent, without delay as much as possible. As such device, a device structure in which a main semiconductor element, such as a power IGBT or a power MOSFET in which a large current flows, and a diode for detecting temperature (temperature sensing diode) are integrally incorporated into the same semiconductor substrate is known.
As illustrated in FIG. 11, it is known that there is a relationship in which a forward direction voltage (hereinafter, forward voltage) of a diode is generally decreased linearly as temperature of an element is increased. By using this property, when a diode layer for detecting temperature is formed on the surface of a semiconductor substrate on which a main semiconductor element (hereinafter, referred to as main element) is mounted with an insulating film therebetween, a temperature change of the main element can be immediately detected as a voltage change. If the detected temperature of the main element exceeds allowable temperature of the main element, an operating current is limited by decreasing a gate voltage of the main element so that the main element can be protected from thermal destruction.
In contrast, a potential difference (Vf: forward voltage) generated between an anode (p region) and a cathode (n region) when passing a forward direction current through a diode for detecting temperature is a sum of a junction voltage Vpn generated in a pn junction part and a voltage drop (I×Rpn) generated by resistance of the p region and the n region. More specifically, Vf=Vpn+(I×Rpn). The resistance value Rpn of the p region and the n region is determined by impurity concentrations of the p region and then region, and thus, if there is variation in the impurity concentrations, variation occurs in the resistance value Rpn of the p region and the n region. As a result, variation occurs in temperature detection accuracy by the diode.
As a manufacturing method of such a diode for detecting temperature, which is integrated with a main element, for example, a manufacturing method in which a p-n junction diode is formed by impurity doping in a polycrystalline silicon layer grown on the surface of a substrate of a main semiconductor element with a silicon oxide film therebetween is known (PTL 1, 2).
In addition, a structure of a diode for detecting temperature (FIG. 6 in PTL 3), in which, when the diode made of respective p-n impurity layers adjacent to each other with a junction therebetween is formed by ion implantation and laser anneal in a polycrystalline silicon layer grown on the surface of a substrate of a main element with an insulating film therebetween, the polycrystalline silicon layer is left in the lower layer part of the p-n impurity layers, is disclosed (PTL 3).
In addition, a document describing details of a manufacturing process of a polysilicon diode is disclosed (PTL 4).
FIGS. 9A through 9E illustrate a conventional manufacturing method of a MOSFET having a diode for detecting temperature (temperature sensing diode). In FIGS. 9A through 9E, hatching indicating cross-sectional surfaces is partially omitted for making the drawing be easily recognized.
As illustrated in FIG. 9A, a polycrystalline silicon layer 104 is formed on the whole area of the surface of a semiconductor substrate 101 having an active region and an inactive region with an insulating film 103 therebetween. Here, a left region in which a main element is formed is the active region, and a right region in which the diode for detecting temperature is formed is the inactive region.
Then, as illustrated in FIG. 9B, by using a photoresist 111 as a mask, the polycrystalline silicon layer 104 and the insulating film 103 on the active region of the semiconductor substrate 101 are selectively removed.
Furthermore, as illustrated in FIG. 9C, by using a photoresist 112 as a mask, boron (B) ions as impurity ions are selectively ion-implanted into the polycrystalline silicon layer 104 on the inactive region of the semiconductor substrate 101 and the active region of the semiconductor substrate 101 to form an impurity ion implantation layer 105a for the diode in the polycrystalline silicon layer 104 and to form an impurity ion implantation layer for the main element in the active region of the semiconductor substrate 101.
Then, as illustrated in FIG. 9D, by using a photoresist 113 as a mask, arsenic (As) ions as impurity ions are selectively ion-implanted into the polycrystalline silicon layer 104 on the inactive region of the semiconductor substrate 101 and the active region of the semiconductor substrate 101 to form an impurity ion implantation layer 106a for the diode in the polycrystalline silicon layer 104 and to form an impurity ion implantation layer for the main element in the active region of the semiconductor substrate 101.
Then, as illustrated in FIG. 9E, by performing thermal treatment to activate the boron ions and the arsenic ions ion-implanted in FIG. 9C and FIG. 9D, a p region 105 and an n region 106 are formed in the polycrystalline silicon layer 104.
Here, in the conventional manufacturing method of a semiconductor device having a MOSFET and a diode for detecting temperature, as illustrated in FIG. 9C and FIG. 9D, when the impurity ions are selectively ion-implanted into the polycrystalline silicon layer 104, in order to shield ion implantation into non-implantation regions other than implantation regions, a step for covering the non-implantation regions with the photoresists 112, 113 becomes necessary. In addition, for promotion of efficiency of manufacturing steps, the p region 105 and the n region 106 configuring the diode for detecting temperature are often formed in the same step as the formation of the main element in the active region of the semiconductor substrate 101 by ion implanting (refer to FIGS. 9C and 9D), as described in paragraphs [0022] to [0023] of PTL 1.
However, the conventional diode for detecting temperature has large variation of the forward voltage Vf.